1. Field of the Invention
The present invention relates to noise filters for controlling electromagnetic interference and, in particular, to a noise filter that attenuates noise by absorbing high-frequency elements.
2. Description of the Related Art
FIG. 26 illustrates a known noise filter 100. The noise filter 100 includes a metal line 101, electrodes 102 connected to both ends of the metal line 101, and a case 103. The electrodes 102 are covered with the case 103. The case 103 is made of a ferrite resin that is a mixture of sintered ferrite powder, which is a magnetic powder, and a resin. The sintered ferrite is not limited to the frequency limit line of a magnetic loss μ″ of a complex permeability represented by μ′-jμ″.
The noise removal range of the noise filter 100 effectively extends into the range of several GHz above 1 GHz. Attenuation in the high-frequency range is expected (Japanese Unexamined Patent Application Publication No. 2000-91125, FIG. 1).
FIG. 27 illustrates another known noise filter 110. An internal conductor 112 is coaxially arranged in and penetrates through a metal cylinder 111 as a cylindrical external conductor. The spacing defined by the metal cylinder 111 and the internal conductor 112 is filled with a composite magnetic material 114 including an Si—Fe based magnetic powder as the major component.
The Si—Fe based magnetic powder as the main component of the composite magnetic material 114 is scaly and has a complex specific permeability of μr′-jμr″ and a complex specific dielectric constant εr′-jεr″.
The noise filter 110, which is a distributed parameter circuit, does not experience the degradation in insertion attenuation characteristics in GHz bands that a noise filter as a lumped parameter circuit typically suffers from due to resonance. The composite magnetic material 114 including the Si—Fe based magnetic powder maintains μr′ in a high frequency range. Along with this, a peak of the μr″ is shifted toward a high frequency region. As a result, the insertion loss characteristic is maintained at a satisfactory level from a MHz band to a GHz band. Reference is made to Japanese Unexamined Patent Application Publication No. 11-273924 (FIG. 1).
FIG. 28 illustrates another known noise filter 120. The noise filter 120 is a low-pass filter that reliably absorbs a high-frequency element in high frequency regions. The noise filter 120 includes a ground electrode 121, a signal line electrode 122, and an insulating base 123. The ground electrode 121 and the signal line electrode 122 are arranged on the insulating base 123. The insulating base 123 is a composite material, including a mixture of a ferromagnetic metal powder and an insulating resin.
The insulating base 123 absorbs unwanted high-frequency elements in a high-frequency range included in a signal conducted by the signal line electrode 122. Reference is made to Japanese Unexamined Patent Application Publication No. 8-78218.
The known noise filter 100 has the following drawbacks. With the case 103 made of a magnetic material surrounding the metal line 101, the noise filter 100 functions as an impedance element with the metal line 101 having an inductance response to the permeability of the case 103. The noise filter 100, connected in series with a transmission line such as a printed circuit board, causes an impedance mismatch, thereby reflecting and thus controlling noise. The complex permeability μ′-jμ″ of the magnetic material forming the case 103 contributes to the impedance of the noise filter 100. Noise controlling is achieved in a frequency range where a magnetic loss μ″ does not occur. In other words, since insertion loss occurs in a low-frequency range, low-frequency passband characteristics are adversely affected.
The noise filters 110 and 120 suffer from the following drawbacks. The noise filters 110 and 120 include a magnetic powder in the composite magnetic material 114 and the insulating base 123. Therefore, the magnetic loss μ″ does not sharply increase, the insertion loss characteristics do not sharply increase, and a large attenuation is not achieved in a frequency range above a certain frequency.